X-ray imaging apparatus have been developed and improved, and are used in a range of applications for a number of 2D (2-dimensional) and 3D (3-dimensional) imaging modalities. In spite of numerous adaptations and ongoing redesign, however, there are some disappointing characteristics of the thermionic emission that is commonly used for X-ray generation. Conventional thermionic or heated-filament X-ray tubes, for example, are characterized by large size, high heat levels, and slow response time, constraining the design of more portable and flexible X-ray systems, including systems used for volume (3D) imaging.
As shown in the schematic diagram of FIG. 1, a traditional thermionic X-ray tube 10 based on the classical heated filament model includes an electron emitter having two metal electrodes formed within a vacuum tube 12. A cathode 14, typically a tungsten filament, is at one end of tube 12, and an anode 16 at the opposite end. The tungsten filament cathode 14 emits electrons when it is heated (for example, to 1,000 degrees C.). X-rays are excited and emitted through a window 18 when electrons internal to the tube are accelerated between the cathode 14 and a target 20, such as a tungsten target, on the anode 16 electrode. Thermionic emission (TE) devices of this type generate significant amounts of heat and often use a rotating anode and active cooling systems to help compensate for thermal effects.
By comparison to thermionic emission devices such as that shown in FIG. 1, field emission (FE) devices offer a number of advantages. FE devices are generally more compact. The field-emission process has thermal characteristics more favorable than those of conventional thermionic apparatus, with emission generated at ambient temperatures. FE devices generate X-rays using a tunneling process, with near-instantaneous emission, well suited to applications using pulsed X-ray emission.
As one type of FE source, carbon nanotubes (CNT) can be used as part of the cathode electrode in an X-ray tube. In place of the single tungsten emitter that provides the cathode for a conventional TE source, the FE device can use an array of structured carbon nanotubes as emitters. The nanotubes emit electrons from their tips instantly when a voltage is applied to them. The use of CNT emitters provides an arrangement that effectively operates as several hundred tiny electron guns that can be fired in rapid succession.
The use of carbon nanotube (CNT) based field emitters is advantaged for more compact design and improved FE behavior. The CNT X-ray sources are generally compact in design and can therefore be packaged closely together, allowing for X-ray source arrays with unique/particular geometries. CNT use enables the design of distributed X-ray sources for medical imaging applications.
There are, however, a number of fabrication hurdles for CNT devices. One problem relates to the need to precondition the X-ray tube components to remove ions that could cause damage to the cathode and shorten cathode working life if proper measures are not taken.
It would be desirable to have a fabrication process that reduces degradation to the cathode during manufacture of a CNT or other type of FE X-ray source.